TypeScript + React 19 Code Review Expert

Expert code reviewer with deep knowledge of React 19's new features, TypeScript best practices, state management patterns, and common anti-patterns.

Review Priority Levels

🚫 Critical (Block Merge)

These issues cause bugs, memory leaks, or architectural problems:

⚠️ High Priority

📝 Architecture/Style

Quick Detection Patterns

useEffect Abuse (Most Common Anti-Pattern)

// ❌ WRONG: Derived state in useEffect
const [firstName, setFirstName] = useState('');
const [fullName, setFullName] = useState('');
useEffect(() => {
  setFullName(firstName + ' ' + lastName);
}, [firstName, lastName]);

// ✅ CORRECT: Compute during render
const fullName = firstName + ' ' + lastName;

// ❌ WRONG: Event logic in useEffect
useEffect(() => {
  if (product.isInCart) showNotification('Added!');
}, [product]);

// ✅ CORRECT: Logic in event handler
function handleAddToCart() {
  addToCart(product);
  showNotification('Added!');
}

React 19 Hook Mistakes

// ❌ WRONG: useFormStatus in form component (always returns false)
function Form() {
  const { pending } = useFormStatus();
  return <form action={submit}><button disabled={pending}>Send</button></form>;
}

// ✅ CORRECT: useFormStatus in child component
function SubmitButton() {
  const { pending } = useFormStatus();
  return <button type="submit" disabled={pending}>Send</button>;
}
function Form() {
  return <form action={submit}><SubmitButton /></form>;
}

// ❌ WRONG: Promise created in render (infinite loop)
function Component() {
  const data = use(fetch('/api/data')); // New promise every render!
}

// ✅ CORRECT: Promise from props or state
function Component({ dataPromise }: { dataPromise: Promise<Data> }) {
  const data = use(dataPromise);
}

State Mutation Detection

// ❌ WRONG: Mutations (no re-render)
items.push(newItem);
setItems(items);

arr[i] = newValue;
setArr(arr);

// ✅ CORRECT: Immutable updates
setItems([...items, newItem]);
setArr(arr.map((x, idx) => idx === i ? newValue : x));

TypeScript Red Flags

// ❌ Red flags to catch
const data: any = response;           // Unsafe any
const items = arr[10];                // Missing undefined check
const App: React.FC<Props> = () => {}; // Discouraged pattern

// ✅ Preferred patterns
const data: ResponseType = response;
const items = arr[10]; // with noUncheckedIndexedAccess
const App = ({ prop }: Props) => {};  // Explicit props

Review Workflow

1. Scan for critical issues first - Check for the patterns in "Critical (Block Merge)" section
2. Check React 19 usage - See react19-patterns.md for new API patterns
3. Evaluate state management - Is state colocated? Server state vs client state separation?
4. Assess TypeScript safety - Generic components, discriminated unions, strict config
5. Review for maintainability - Component size, hook design, folder structure

Reference Documents

For detailed patterns and examples:

• [react19-patterns.md](references/react19-patterns.md) - React 19 new hooks (useActionState, useOptimistic, use), Server/Client Component boundaries
• [antipatterns.md](references/antipatterns.md) - Comprehensive anti-pattern catalog with fixes
• [checklist.md](references/checklist.md) - Full code review checklist for thorough reviews

State Management Quick Guide

TanStack Query Anti-Pattern

// ❌ NEVER copy server data to local state
const { data } = useQuery({ queryKey: ['todos'], queryFn: fetchTodos });
const [todos, setTodos] = useState([]);
useEffect(() => setTodos(data), [data]);

// ✅ Query IS the source of truth
const { data: todos } = useQuery({ queryKey: ['todos'], queryFn: fetchTodos });

TypeScript Config Recommendations

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitReturns": true,
    "exactOptionalPropertyTypes": true
  }
}

noUncheckedIndexedAccess is critical - it catches arr[i] returning undefined.

Immediate Red Flags

When reviewing, flag these immediately: